This invention relates to solid state lasers, and in particular to a method and apparatus of generating at room temperature one or more wavelengths in the infrared part of the spectrum using high concentration Holmium-doped fluoride crystals to maximize resonant pump absorption.
It is well known that the trivalent holmium ion (Ho3+) is capable of producing stimulated emission at several different wavelengths across the infrared, from 0.75 to just under 4.0 xcexcm. For the purpose of generating longer wavelengths, fluoride crystals are a preferred host for the holmium ion because the energy levels are spaced sufficiently apart within the different manifolds to mitigate against rapid multiphonon non-radiative transitions which would otherwise inhibit fluorescence at wavelengths longer than about 3 xcexcm. Thus, while the Ho transition near 2.9 xcexcm has been made to lase in many different crystals including oxides and garnets, only fluorides exhibited stimulated emission beyond 3 xcexcm. It is further known that because of the rich energy level structure of Ho, a multiplicity of wavelengths can be generated through sequential transitions between intermediate levels.
One of the most interesting Ho transitions is the one near 4 xcexcm between the 5I6 and 5I5 levels. There are very few active ion-host crystal combinations that have been successfully lased this far into the infrared, and none that have demonstrated operation levels substantially greater than a few millijoules at or near room temperature. As will be described below, stimulated emission at 3.9 xcexcm was previously achieved by Ho:YLF but under conditions that severely limit prospects for further energy and power scaling to levels that are of interest.
The main issue limiting laser action at 3.9 xcexcm in Ho-doped crystals, including most known fluorides, is the long fluorescence lifetime of the lower 5I6 laser level coupled with the self-terminating nature of the 5I5xe2x86x925I6 transition. The long 5I6 lifetimexe2x80x94up to a few milliseconds for most fluoride materialsxe2x80x94limits the repetition rate of the corresponding laser transition, whereas the much shorter lifetime of the upper 5I5 levelxe2x80x94typically, no more than a few 10""s of microseconds, results in an effective three-level system for the laser transition. While it is known in the art that cooling of a three level laser medium can be used to more easily achieve and sustain inversion, this approach is generally considered unattractive for practical laser systems because of added complexity and weight. It has further been recognized that an alternative way to overcome an unfavorable lifetime ratio is through use of resonant pumping, whereby the upper laser level is directly excited by a narrow band source with frequency selected or tuned to match an absorption line that is dynamically connected to the upper level of the desired transition. When the resonant pump source also has a very short pulse duration (typically about 100 nanoseconds) it is said to xe2x80x9cgain switchxe2x80x9d the particular transition, in much the same way Q-switching a laser oscillator produces short duration pulses.
Resonant pumping for the purpose of generating mid-infrared wavelengths from activator ions in various hosts has often been employed in the prior art. For example, in the invention disclosed in U.S. Pat. No. 5,200,966 to Esterowitcz and Stoneman, the 4I11/2 upper laser state of the erbium ion was directly pumped with a pump beam at a wavelength of about 970 nm, causing the erbium ion to produce laser emission at substantially 2.8 xcexcm, corresponding to the 4I11/2xe2x86x924I15/2 laser transition, with high efficiency at room temperature. Because high power diode laser arrays with wavelengths in the 950-980 nm range have recently become more available, there have been several successful efforts demonstrating diode pumped, power scalable cw operation from Er-doped lasers. However, pulsed operation has been more elusive at or near 3 xcexcm, even under seemingly favorable resonant pumping conditions. In another example, U.S. Pat. No. 4,330,763 to Esterowitcz and Kruer taught use of resonant pumping from a laser source at 2.06 xcexcm to achieve inversion on the 7F3xe2x86x927F5 line at 4.1 xcexcm from terbium-doped YLF. A large ratio of non-radiative to radiative decay rates in this gain material discriminates against broad-band pumping, but allows the use of resonant, narrow-band excitation to produce laser action.
Heretofore, Holmium-doped lasers have also been made which are capable of pulsed operation in the infrared region of the spectrum upon resonant pumping by radiation from Nd:YAG lasers with output near 1 xcexcm. In particular, pulsed emission at or near 3 xcexcm from Ho-doped garnets such as YAG, GGG and YALO was described wherein co-doping with suitable activator ion such as praseodymium (Pr) was utilized to allow resonant pumping near 1 xcexcm. For example, Anton in U.S. Pat. No. 5,070,507 describes a laser system wherein a Nd-doped laser operating on a non-standard line of 1.123 xcexcm is used to pump holmium laser to produce a moderately high energy output pulse at about 3 xcexcm. Key to the invention by Anton was the incorporation of holmium ion with concentrations in excess of 15% (atomic percent) and a much lower praseodymium (Pr) concentration (on the order of 0.01%). The higher Ho concentration allowed preferential lasing on the 2.94 xcexcm line in Ho-doped garnet crystals upon pumping with the 1.12 xcexcm output of a Nd:YAG laser, whereas the Pr ion served to quench the lifetime of the lower 5I7 laser level, thereby breaking the bottleneck of the normally self-terminating 5I5xe2x86x925I6 transition.
In the early demonstrations of the long wavelength transitions in Ho3+-doped YLF using resonant pumping of the 5S2 manifold with short pulse green lasers, laser action on the 3.9 xcexcm line was achieved as part of a sequence with other transitions, a process known in the art as cascade lasing. Specifically, using a frequency-doubled short pulse (20 ns) Nd:glass laser operating at 535 nm to pump a 1% Ho:YLF crystal, the two-line 5S2xe2x86x925I5, 5I5xe2x86x925I6(1.392 xcexcm, 3.914 xcexcm) and 5S2xe2x86x925I5, 5I5xe2x86x925I7 (1.392 xcexcm, 1.673 xcexcm) cascade transitions were successfully lased at room temperature (see L. Esterowitz, R. C. Eckardt and R. E. Allen, Appl. Phys. Lett., 35,236, (1979)). Three-step laser transitions, for example at 3.4 xcexcm, 3.9 xcexcm and 2.9 xcexcm were also reported (see R. C. Eckart, L. Esterowitz and Y. P. Lee, Procs. Int""l Conf. Lasers, pp. 380 (1981)) in Ho:YLF using the longer 1 xcexcs pulse from a pulsed dye laser tuned to 535.5 nm. These and similar results were further described in U.S. Pat. No. 4,321,559 to Esterowitz and Eckardt. A key feature in these early descriptions of resonantly pumped cascade lasing was that cascade processes, whereby one laser transition sequentially pumps a lower laser transition in the same material, could be viewed as one form of resonant self-pumping. By causing population inversion to occur sequentially, cascade laser action can therefore improve the efficiency of laser transitions between intermediate manifolds, as well as produce radiation consisting of two or more wavelengths. In the case of short pulse green laser excitation of the high lying 5S2 state, cavity optics can be selected to preferentially lase a given sequence of transitions. For example, by using one set of coated optics, the excited 5S2 state population could be directly transferred to the intermediate 5I5 level, which then serves as the upper level for a subsequent 3.9 xcexcm laser transition to the 5I6 level. A different set of cavity mirrors cause the second lasing step to occur on the 1.7 xcexcm 5I5xe2x86x925I7 line.
Yet, although prior art describing the advantages of resonant pumping and multi-wavelengths cascade lasing was related nearly two decades ago, to date no practical Ho-doped laser has been constructed with one output wavelength near either the 2.9xcexc or 3.9 xcexcm lines, using principles taught by Esterowitcz and Eckardt. One problem with prior art systems based on resonant pumping is that they require a laser with a wavelength tuned closely to an appropriate absorption band of the laser material. For example, in the case the Ho ion, lasing at 3.9 xcexcm was previously obtained only as part of a sequence of cascade transitions, by resonantly pumping the 5I8 ground state to the 5S2, 5F4 level. To increase the pumping efficiency, the green beam had to be tuned close to the appropriate absorption peak, which in fluorides is near 535 nm. This wavelength matches up poorly with most readily available commercial lasers, which is one of the factors precluding practical application of such cascade lasers to date. Similarly, the methods and system disclosed by Anthon for generating 2.9 xcexcm radiation from Ho-doped garnets, while recognizing the benefits to improved efficiencies that could be obtained by increasing holmium concentrations, still required a pump laser tuned to 1.1 xcexcm, which is a difficult wavelength to obtain from a practical laser system, especially if short pulse operation is desired as well. Thus, even if pump lasers with wavelengths suitable for pumping holmium could be constructed, other conditions on the pulse duration, energy, repetition rate, and beam quality may place additional limitations on practical implementations of the infrared laser system with the output power, output wavelengths and efficiency desired.
It is therefore one object of the present invention to disclose a means for achieving efficient room temperature laser operation at 3.9 xcexcm from a holmium-doped fluoride crystal pumped by a practical pulsed source tuned to a resonance, and with pulse duration short enough to allow population inversion between the upper 5I5 level and the long lived 5I6 lower laser level.
It is another object of the invention to disclose a pulsed Ho-doped laser operatively configured as a 2.9 xcexcm or 3.9 xcexcm downconverter for a shorter wavelength laser that is available as a commercial source. Examples of such sources include the 532 nm from frequency-doubled Nd:YAG, Nd:Vanadate, or other Nd-doped systems a Ti:sapphire or Cr:LiSAF laser tuned to about 890 nm, a fosterite or fiber Raman laser operating near 1.2 xcexcm.
It is an additional object to be able to efficiently accomplish said downconversion utilizing short pulse (nanosecond) pump lasers, thereby gain switching the transitions near 2.9 and/or 3.9 xcexcm so as to produce short pulses at these infrared wavelengths.
It is still another object to generate one or more different wavelengths in the infrared between 750 nm and 4 xcexcm, but specifically including the wavelengths near 2.9 and/or 3.9 xcexcm, utilizing resonant pumping of a holmium-doped fluoride crystal with a shorter wavelength pump laser.
It is a further object to provide a method and system for generating said output wavelengths alone or in a cascade with other infrared wavelengths at output energies scalable to over 10 millijoules and with repetition rates scalable to over 10 Hz.
It is yet another object to disclose methods for generating energy scalable longer infrared wavelengths at room temperature using a resonant pump source with pulse duration that is sufficiently long to enable efficient pumping even from end-pumped configurations. It is therefore a special object to be able to operate the pump laser at energy densities that are well above the threshold for sustained laser oscillation while staying below damage thresholds to sensitive IR coatings. In various embodiments of the invention such pump sources may include free running, or long pulse tunable Cr:LiSAF or Ti:sapphire lasers, frequency-doubled Nd-doped lasers, Raman fiber lasers and high power, quasi-cw semiconductor laser arrays.
In accordance with the above objectives, system and method is disclosed for generating at least one long infrared wavelength from a holmium-doped fluoride laser source pumped by a resonant pulsed narrow-band source. The invention includes pump sources with short enough pulse durations to gain switch a particular transition and also pump sources with long pulses but sufficiently high energy dsensity to overcome the saturation density associated with the transition. Of particular importance to the present invention are techniques for selecting the holmium concentration so as to optimize absorption at a wavelength that is available as a practical commercial laser source. In preferred embodimets of the invention the particular wavelengths of 3.9 xcexcm and 2.9 xcexcm are generated alone or in sequence with each other or with other wavelengths including but not limited to 1.4 xcexcm, 2.4 xcexcm and 2.0 xcexcm. Pump wavelengths include 532 nm, such as is available from stanfdard Nd-doped lasers, 890 nm from Cr:LiSAF, Ti:sapphire, or diode laser arrays and 1.2 xcexcm from, for example fosterite or Raman fiber lasers.